Water heating systems are employed in both domestic and industrial environments for space heating and/or for supplying hot water for washing purposes. Water heating systems employ a primary heater, which is typically a gas or oil fired boiler. Where the heating system is to be used in a building for space heating purposes, the system comprises a network of space heaters. These space heaters are heat exchangers, and are generally referred to as “radiators”. The primary heater is coupled to the radiators in a closed loop by means of a network of pipes, which includes a primary pipe circuit coupled to the boiler, and branch pipes extending from the primary pipe circuit to the radiators. The primary heater is activated to heat water in the primary pipe circuit, which is pumped around the primary pipe circuit and fed to the radiators by the branch pipes.
As the heated water passes around the primary pipe circuit and through the radiators, heat is transferred to the air via the radiators, to warm the building. The heated water flowing from the boiler into the primary pipe circuit is known as the “primary flow”, and the water returning to the boiler through the pipe circuit is known as the “primary return”. As a result of heat transfer to the radiators, and thermal losses in the pipe network, the water returning to the boiler (the primary return) is at a lower temperature than the water exiting the boiler (the primary flow).
Where the water heating system is also used to provide a source of hot water for washing purposes, a hot water storage vessel or tank is provided in a secondary circuit which receives water from the primary pipe circuit. Water from the primary circuit enters the secondary circuit and passes through a coiled pipe located in the tank. This serves for heating a volume of ambient temperature water that has been charged into the tank. The water in the tank is thus indirectly heated by the primary flow, and the heated water that is outputted from the secondary circuit containing the tank is referred to as the “secondary flow”.
Other systems are known in the industry, in particular those of the type which provide a supply of hot water for washing purposes on-demand. In systems of this type, cold water is supplied to the boiler and is heated and supplied directly to a tap (faucet) or taps for discharge. The storage tank is thus dispensed with. Boilers of the type that are used in these systems are typically referred to in Europe as “combination” or “combi” boilers. These systems also serve for heating water in a closed pipe circuit for space heating purposes in the fashion described above. However, other more simple systems are known, which serve merely for providing hot water for washing purposes.
The water which is utilised in water heating systems for space heating purposes contains a relatively large proportion of dissolved air, which is a mixture of a number of different gases. The primary constituents of air are Nitrogen (˜78%) and Oxygen (˜21%). It has been calculated that the typical proportion of dissolved air in water is around 0.023 g/kg (or around 0.0227 g/liter) for water at a temperature of 25° C. In terms of volume, it is estimated that water at room temperature will contain up to around 2.5% by volume of dissolved air. In other words, 100 liters of water will contain up to around 2.5 liters of dissolved air.
However, the capacity of water to absorb dissolved air decreases with temperature (and increases with pressure). Consequently, when the water in a water heating system is heated, a portion of the air which is dissolved in the water comes out of solution. The gases coming out of solution from the water can form bubbles which are mixtures of different gases. These gases impair the performance of the water heating system.
In particular, with Nitrogen being the largest constituent of air, it has been found that microscopic bubbles of Nitrogen can line the network of pipes and radiators, forming a thermal barrier which reduces the effective transfer of heat to the radiators, thereby impairing performance. Additionally, the bubbles lining the pipes and radiators increase surface friction, and thus the resistance to flow of water through the pipes and radiators. This can result in impaired heating performance, as the flow of heated water to radiators further along the heating system from the boiler may not be sufficient to adequately heat the parts of the building served by those radiators. This typically results in the boiler being operated at a higher capacity than would otherwise be required, leading to increased energy usage and thus costs, and a resultant increase in wear and tear on the boiler. Additionally, a pump used to force the water around the pipes has to operate at a higher work rate, also leading to increased energy usage, costs, and wear and tear. Furthermore, the presence of oxygen in the system can lead to corrosion of the radiators, which are typically of a mild steel material. Finally, the bubbles of gas can become entrained in the water flowing around the primary pipe circuit and the radiators, resulting in unwanted noise.
Attempts have been made to address these problems by removing dissolved air from the water employed in water heating systems. These have included positioning de-aeration vessels (which are typically cylindrical, conical or having a conical base) in the primary pipe circuit so that the hot water flowing around the pipe circuit (and into a secondary circuit containing a hot water storage tank, if used) has to pass through the vessel. Vessels of this type seek to increase the velocity of the fluid, generating a vortex which, as a consequence, reduces the pressure of the fluid. Reducing the pressure of a fluid also reduces its capacity to retain dissolved air. The aim of such vessels is therefore to reduce the pressure of the water so that dissolved air comes out of solution, and can be directed out of the vessel along a dip pipe which communicates with the atmosphere.
In practice however, none of the vessels which have been developed have been found to provide an effective removal of the air which comes out of solution when the water is heated. In particular, it has been found that heating systems employing such vessels do not provide a sufficiently significant improvement in performance to address the deficiencies outlined above. Additionally, it has been found that a significant problem of corrosion of the mild steel radiators still exists, indicating that there remains a significant volume of oxygen in the water in the heating system. It is believed that this is due not only to inefficient removal of dissolved air from the water, but also a peculiar problem associated with the large proportion of Nitrogen found in air. In particular, it is believed that the microscopic bubbles of Nitrogen which come out of solution from the water attract oxygen, and carries the oxygen out of the vessel into the radiators.
One reason for the prior vessels performing inadequately has been found to be their shape. Specifically, in the existing vessels, it has been found that, under many operating conditions, a vortex is set-up which is of a shape that results in a pressure drop which is not consistent or sufficient to provide an effective removal of gases from the water. Also, the microscopic bubbles of Nitrogen which come out of solution from the water do not have sufficient buoyancy to exit the vessel through the dip pipe against the force of the water flowing through the vessel. Furthermore, it has been found that the vortex which is formed actually tends to draw additional air into the vessel, rather than allowing exit of gases that have come out of solution, thus exacerbating the problem. Conically shaped vessels and vessels with conical portions have been found to result in the formation of unstable vortices, and resultant poor performance. Turbulence at the outlet of the vessels can also disrupt the flow and formation of a stable vortex.
It is therefore amongst the objects of the present invention to obviate or mitigate at least one of the foregoing disadvantages.